Raising CYA
One pound of granular stabilizer per 10,000 gallons raises CYA about 12 ppm. It dissolves slowly and reads low for days, which is how people end up at 150 ppm: they retest after a day, see nothing, and add more. Add the dose, wait a full week, then test.
Lowering CYA
Nothing you can pour in removes it. The only way down is replacing water. To go from 100 to 50 you drain half. Test your fill water too, and skip the CYA reducers sold in stores; independent tests show they rarely move the number. If you use trichlor tablets, every tablet adds CYA, so switch to liquid chlorine once you're at target.
Why it changes your chlorine target
Higher CYA means you need more free chlorine to get the same sanitizing power: roughly 7.5% of CYA as a daily minimum, and 40% of CYA to shock. The shock calculator works that out from your CYA reading.